1. Field of the Invention
The present invention relates to radio receivers generally and more particularly to a GPS satellite receiver employing a balanced mixer and a half-frequency local oscillator for down converting the frequency of a received satellite signal in a single step and a hard limiter for converting from analog-to-digital form the down-converted satellite signal.
2. Description of the Prior Art
The NAVSTAR Global Positioning System (GPS) is a United States Defense Department satellite-based radio-navigation system transmitting information from which extremely accurate navigational information can be computed including the time, the user's three-dimensional position anywhere on or near the Earth, and the user's three-dimensional velocity. When fully operational, the GPS is planned to employ 18 satellites evenly dispersed in three, inclined, 12-hour circular orbits chosen to insure continuous 24-hour coverage worldwide. Each satellite carries extremely accurate cesium and rubidium vapor atomic clocks providing timing information. Additionally, each satellite is provided clock correction and orbital information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band carrier signals including an L1 signal having a frequency of 1575.42 MHz (also referred to as 1540 f0 where f0 is 1.023 MHz) and an L2 signal having a frequency of 1227.6 MHz (1200 f0). The L1 and L2 signals are biphase modulated by pseudo-random noise (PRN) codes. The PRN codes facilitate multiple access. Since each satellite uses different PRN codes, a signal transmitted by a particular satellite can be selected by generating and matching (correlating) the corresponding PRN code pattern. Additionally, the PRN codes facilitate signal transit time measurements which can be made by measuring the phase shift required to match the code. Both of the carrier signals (L1 and L2) are modulated by a PRN code which is referred to as a precision (p) code. The p PRN code, which is intended for military purposes, is a relatively long, fine-grained, precision code having a clock rate of 10.23 MHz (10 f0). The L1 carrier signal is additionally modulated by a PRN code which is referred to as a clear/acquisition (C/A) code. The C/A PRN code, which is intended for rapid signal acquisition and for commercial purposes, is a relatively short, coarse-grained code having a clock rate of 1.023 MHz (f0) and a code length of 1023 bits (one ms). A full bit (chip) of C/A PRN code, phase delay corresponds to a distance of 293 meters. In addition to the PRN codes, both of the signals (L1 and L2) are, continuously, biphase modulated by a 50 bit per second, 1500 bit long, navigation data bit stream. The navigation data bit stream includes information as to the status and emphemeris of all satellites, parameters for computing the particular satellite clock, and corrections for atmospheric propagation delays.
Commonly, prior-art-type GPS receivers receive and down convert, in multiple steps, the frequency of an (L1 or L2) satellite signal. A typical prior-art-type GPS satellite receiver is disclosed in the paper by Phil Ward which is entitled "An Advanced NAVSTAR GPS Multiplex Receiver" and which was presented at the IEEE PLANS '80 1980 Position Location and Navigation Symposium, Atlantic City, N.J., Dec. 9, 1980. The above-identified receiver down converts the frequency of a satellite signal in two steps. For the first down-conversion step, the above-identified receiver employs an RF filter, a protective-type limiter, an RF amplifier and a first mixer all connected in cascade from an L-band antenna and a first-local-oscillator-signal-generating oscillator connected to also drive the mixer. In addition to attenuating off-frequency signals, the RF filter is operative to attenuate noise at the first-mixer-image frequency (the frequency other than the L1 or L2 frequency which differs from the frequency of the first-local-oscillator signal by the intermediate frequency). As indicated, the limiter is of the protective type to pass unaltered all signals except those of such unusually high level as present a danger of damaging the RF amplifier. The RF amplifier amplifies received signals so as to establish the receiver noise figure. The first-local-oscillator develops the first-local-oscillator signal to have a frequency which differs from the L1 or L2 frequency by the intermediate frequency to cause the first mixer to down convert to the intermediate frequency the frequency of the satellite signal.
For the second down-conversion step the above-identified receiver employs an IF filter, an IF amplifier, a second mixer, a filter, and a first AGC amplifier all connected in cascade from the first mixer output and a second local oscillator connected to also drive the mixer to develop at the AGC amplifier output the down-converted satellite signal.
Commonly, prior-art-type GPS satellite receivers use analog techniques to remove from the down-converted satellite signal dopler-shift information and PRN-code modulation. For removing the satellite-signal dopler-shift information, the above-identified receiver employs a frequency-controlled synthesizer for generating a signal representing the dopler-shift information and a mixer connected to multiply (a signal developed from) the down-converted satellite signal by (a signal developed from) the synthesized dopler-shift signal. For removing the PRN-code modulation, the above-identified receiver employs a phase-controlled synthesizer for generating a signal representing the pertinent PRN code and a mixer connected to multiply (a portion of) a signal developed by the dopler mixer by the synthesized PRN-code signal. Also employed is a bandpass filter and a second AGC amplifier connected in cascade from the PRN-code-mixer output.
Additionally, prior-art-type GPS satellite receivers commonly use a substantially analog, Costas-type loop for phase locking the receiver to the down-converted satellite signal. For this purpose, the above-identified receiver employs an oscillator for generating a pair of down-converted satellite-signal-frequency signals which are in phase quadrature and a pair of mixers connected to multiply the signal developed by the second AGC amplifier each by a respective one of the phase-quadrature signals to develop an in-phase (I) signal and a quadrature (Q) signal. The receiver also employs analog filters and amplifiers connected to filter and amplify the in-phase (I) and quadrature (Q) signals and a multiplexor, an analog-to-digital converter and an integrate-and-dump-type digital filter connected to convert to digital form the amplified and filtered in-phase (I) and quadrature (Q) signals. Further, the receiver employs a microcomputer connected to receive (signals developed from) the digitized in-phase (I) and quadrature (Q) signals and to control both the frequency of the dopler-shift-signal synthesizer and the phase of the PRN-code-signal synthesizer. From the relative velocity information (obtained from controlling the dopler-shift-signal synthesizer), the transit time information (obtained from controlling the phase of the PRN-code-signal synthesizer), and the navigation bit stream information (obtained from the in-phase (I) signal), the microcomputer computes navigational information.
Finally, the above-identified receiver employs tau-dither PRN-code-phase-matching-error minimizing circuitry which toggles the PRN-code-signal synthesizer one half bit early and one half bit late.
Another prior-art-type receiver is disclosed in an article by Roger McLean and Quyen D. Hua of Stanford Telecommunications, Inc., 1195 Bordeaux Drive, Sunnyvale, Calif. 94086, which is entitled "An Advanced Microprocessor-Controlled GPS Time Transfer System" which was presented at the 1983 National Aerospace Meeting of the Institute of Navigation, in Arlington, Va. on Mar. 22-25, 1983. This latter receiver employs circuitry which, in addition to generating a signal representing the punctual PRN code, generates a signal which represents the PRN code one half bit early and another signal which represents the PRN code one half bit late. An analog down-converted satellite signal is multiplied by a signal representing the difference between the early and late PRN-code signals in a tri-state mixer to develop a signal, which when filtered and amplified, is used by a microcomputer to minimize PRN-code-phase-matching error.
Although effective, it is no doubt apparent that the above-identified receivers are disadvantageous in that they are relatively complex, expensive and bulky.
In addition to the two above-identified articles, the reader may find of interest the article by Kai P. Yin, Ralph Eschenbach and Frank Lee which is entitled "Land Navigation With A Low Cost GPS Receiver" and which was reprinted from the National Telecommunication Conference, Nov. 30-Dec. 4, 1980, by the IEEE and an article by John W. Murphy and Michael D. Yakes of Collins Government Avionics Division, Cedar Rapids, Iowa 52498, which is entitled "Collins Avionics NAVSTAR GPS Advanced Digital Receiver" and which was presented at the Institute of Navigation's 1983 National Aerospace Meeting, Arlington, Va. on Mar. 22-25, 1983.